WASHINGTON -- Bowing to rank-and-file pressure, the Democratic chairmen of the Senate and House tax-writing committees have agreed to propose a tax cut for middle-income taxpayers in October and to push for its adoption next year.

Because the legislation is expected to meet pay-as-you-go requirements of last year's budget agreement between President Bush and Congress by sharply increasing taxes for upper-income Americans, however, it appears certain to be vetoed with scant chance of an override.

But Democratic leaders in both chambers have decided to press the legislation to sharpen differences on tax policy with the Bush administration in a presidential election year.

A reluctant Rep. Dan Rostenkowski, D-Ill., chairman of the House Ways and Means Committee, and Sen. Lloyd M. Bentsen, D-Texas, chairman of the Senate Finance Committee, have agreed to develop a tax bill and plan to introduce the legislation next month. Committee consideration, however, will be put off until Congress returns in 1992.

House Majority Leader Richard A. Gephardt, D-Mo., for example, has planned a news conference today to publicize a new study that concludes that tax cuts enacted in 1981 have been a huge windfall for the top 1 percent of taxpayers, while adding to the tax burdens of middle-income families.

Rostenkowski has also said that he believes that there is a risk in pushing a tax bill: Bush and his allies may be able to attach their long-sought reduction in capital gains taxes, which would earmark most benefits for wealthy investors, to any tax bill as it proceeds through Congress.